The present invention relates generally to a sensor with a dynamic latch and more particularly to a sensor with a dynamic memory to allow for full performance with low power and multiplexed applications.
Low power sensors are used in a variety of applications where the conservation of supply current is a priority, such as in battery-operated equipment. An exemplary low power sensor is a magnetic sensor which uses, for example, a Hall effect element for the transducer, and which provides broad versatility in position sensing applications. For example, a Hall sensor could sense the proximity of a small magnet in a cellular flip-phone. When the cover is opened, the sensor signals the cell phone controller to begin the appropriate actions desired by the user, such as illumination of the keyboard. Subsequently, closure of the cover terminates the phone call and returns the phone to a low power, standby state awaiting incoming calls. Typically, the Hall effect sensor also provides a "hysteresis" function for such position-sensing applications, toggling its magnetic threshold between an "Operate" (ON) level and a "Release" (OFF) level. This function tends to eliminate the output uncertainty arising from vibrations or electrical noise as the sensor approaches its switching point.
Ideally, the standby state of sensors in such a system requires "zero" current for optimum battery life. Present approaches for power conservation employ on-chip clock circuitry that momentarily activates the sensing circuitry for a small percentage of the duty cycle. Thus, the average current utilized by these devices is much lower than a similar device with continuous excitation. However, the clock circuit and some form of memory to "recall" the last state of the sensor need continuous power to achieve the aforementioned hysteresis function.
A photodetector circuit described by Johnson et al. in U.S. Pat. No. 5,151,591 and a Hall circuit implemented by Vig in U.S. Pat. No. 5,619,137 are two examples of strobed sensors in which power is periodically applied by a system controller. However, the designs according to these patents do not have the necessary elements for full performance with complete interruption of the power source.
The present invention allows complete power-down during OFF periods by providing a "dynamic" memory (or a memory that "refreshes" with momentary power application).